NAME

SYNOPSIS

DESCRIPTION

Socat is a command line based utility that establishes two bidirectional byte
streams and transfers data between them. Because the streams can be constructed
from a large set of different types of data sinks and sources
(see address types), and because lots of
address options may be applied to the streams, socat can
be used for many different purposes.

Filan is a utility that prints information about its active file
descriptors to stdout. It has been written for debugging socat, but might be
useful for other purposes too. Use the -h option to find more infos.

Procan is a utility that prints information about process parameters to
stdout. It has been written to better understand
some UNIX process properties and for debugging socat, but might be
useful for other purposes too.

The life cycle of a socat instance typically consists of four phases.

In the init phase, the command line options are parsed and logging is
initialized.

During the open phase, socat opens the first address and afterwards the
second address. These steps are usually blocking; thus, especially for complex address types like socks,
connection requests or authentication dialogs must be completed before the next
step is started.

In the transfer phase, socat watches both streams' read and write file
descriptors via
select()
, and, when data is available on one side and
can be written to the other side, socat reads it, performs newline
character conversions if required, and writes the data to the write file
descriptor of the other stream, then continues waiting for more data in both
directions.

When one of the streams effectively reaches EOF, the closing phase
begins. Socat transfers the EOF condition to the other stream,
i.e. tries to shutdown only its write stream, giving it a chance to
terminate gracefully. For a defined time socat continues to transfer data in
the other direction, but then closes all remaining channels and terminates.

OPTIONS

Socat provides some command line options that modify the behaviour of the
program. They have nothing to do with so called
address options that are used as parts of address specifications.

-V

Print version and available feature information to stdout, and exit.

-h | -?

Print a help text to stdout describing command line options and available address
types, and exit.

-hh | -??

Like -h, plus a list of the short names of all available address options. Some options are
platform dependend, so this output is helpful for checking the particular
implementation.

-hhh | -???

Like -hh, plus a list of all available address option names.

-d

Without this option, only fatal and error messages are generated; applying
this option also prints warning messages. See DIAGNOSTICS
for more information.

-d -d

Prints fatal, error, warning, and notice messages.

-d -d -d

Prints fatal, error, warning, notice, and info messages.

-d -d -d -d

Prints fatal, error, warning, notice, info, and debug
messages.

-D

Logs information about file descriptors before starting the transfer phase.

-ly[<facility>]

Writes messages to syslog instead of stderr; severity as defined with -d
option. With optional <facility>, the syslog type can
be selected, default is "daemon". Third party libraries might not obey this
option.

-lf <logfile>

Writes messages to <logfile> [filename] instead of
stderr. Some third party libraries, in particular libwrap, might not obey
this option.

-ls

Writes messages to stderr (this is the default). Some third party libraries
might not obey this option, in particular libwrap appears to only log to
syslog.

-lp<progname>

Overrides the program name printed in error messages and used for
constructing environment variable names.

-lu

Extends the timestamp of error messages to microsecond resolution. Does not
work when logging to syslog.

-lm[<facility>]

Mixed log mode. During startup messages are printed to stderr; when socat
starts the transfer phase loop or daemon mode (i.e. after opening all
streams and before starting data transfer, or, with listening sockets with
fork option, before the first accept call), it switches logging to syslog.
With optional <facility>, the syslog type can be
selected, default is "daemon".

-lh

Adds hostname to log messages. Uses the value from environment variable
HOSTNAME or the value retrieved with uname() if HOSTNAME is not set.

-v

Writes the transferred data not only to their target streams, but also to
stderr. The output format is text with some conversions for readability, and
prefixed with "> " or "< " indicating flow directions.

-x

Writes the transferred data not only to their target streams, but also to
stderr. The output format is hexadecimal, prefixed with "> " or "< "
indicating flow directions. Can be combined with
-v
.

-b<size>

Sets the data transfer block <size> [size_t].
At most <size> bytes are transferred per step. Default is 8192 bytes.

-s

By default, socat terminates when an error occurred to prevent the process
from running when some option could not be applied. With this
option, socat is sloppy with errors and tries to continue. Even with this
option, socat will exit on fatals, and will abort connection attempts when
security checks failed.

-t<timeout>

When one channel has reached EOF, the write part of the other channel is shut
down. Then, socat waits <timeout> [timeval] seconds
before terminating. Default is 0.5 seconds. This timeout only applies to
addresses where write and read part can be closed independently. When during
the timeout interval the read part gives EOF, socat terminates without
awaiting the timeout.

-T<timeout>

Total inactivity timeout: when socat is already in the transfer loop and
nothing has happened for <timeout> [timeval] seconds
(no data arrived, no interrupt occurred...) then it terminates.
Useful with protocols like UDP that cannot transfer EOF.

-u

Uses unidirectional mode. The first address is only used for reading, and the
second address is only used for writing (example).

-U

Uses unidirectional mode in reverse direction. The first address is only
used for writing, and the second address is only used for reading.

-g

During address option parsing, don't check if the option is considered
useful in the given address environment. Use it if you want to force, e.g.,
appliance of a socket option to a serial device.

-L<lockfile>

If lockfile exists, exits with error. If lockfile does not exist, creates it
and continues, unlinks lockfile on exit.

-W<lockfile>

If lockfile exists, waits until it disappears. When lockfile does not exist,
creates it and continues, unlinks lockfile on exit.

-4

Use IP version 4 in case that the addresses do not implicitly or explicitly
specify a version; this is the default.

-6

Use IP version 6 in case that the addresses do not implicitly or explicitly
specify a version.

ADDRESS SPECIFICATIONS

With the address command line arguments, the user gives socat instructions and
the necessary information for establishing the byte streams.

An address specification usually consists of an address type
keyword, zero or more required address parameters separated by ':' from the keyword and
from each
other, and zero or more address options separated by ','.

The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some
keywords there exist synonyms ('-' for STDIO, TCP for TCP4). Keywords are case
insensitive.
For a few special address types, the keyword may be omitted:
Address specifications starting with a number are assumed to be FD (raw file
descriptor) addresses;
if a '/' is found before the first ':' or ',', GOPEN (generic file open) is
assumed.

The required number and type of address parameters depend on the address
type. E.g., TCP4 requires a server specification (name or address), and a port
specification (number or service name).

Zero or more address options may be given with each address. They influence the
address in some ways.
Options consist of an option keyword or an option keyword and a value,
separated by '='. Option keywords are case insensitive.
For filtering the options that are useful with an address
type, each option is member of one option group. For
each address type there is a set of option groups allowed. Only options
belonging to one of these address groups may be used (except with option -g).

Address specifications following the above schema are also called single
address specifications.
Two single addresses can be combined with "!!" to form a dual type
address for one channel. Here, the first address is used by socat for reading
data, and the
second address for writing data. There is no way to specify an option only once
for being applied to both single addresses.

Usually, addresses are opened in read/write
mode. When an address is part of a dual address specification, or when
option -u or -U is used, an address might be
used only for reading or for writing. Considering this is important with some
address types.

With socat version 1.5.0 and higher, the lexical analysis tries to handle
quotes and parenthesis meaningfully and allows escaping of special characters.
If one of the characters ( { [ ' is found, the corresponding closing
character - ) } ] ' - is looked for; they may also be nested. Within these
constructs, socats special characters and strings : , !! are not handled
specially. All those characters and strings can be escaped with \ or within ""

ADDRESS TYPES

This section describes the available address types with their keywords,
parameters, and semantics.

CREATE:<filename>

Opens <filename> with
creat()
and uses the file
descriptor for writing.
This address type requires write-only context, because a file opened with
creat
cannot be read from.
Flags like O_LARGEFILE cannot be applied. If you need them use
OPEN with options
create,create.
<filename> must be a valid existing or not existing path.
If <filename> is a named pipe,
creat()
might block;
if <filename> refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options:
mode,
user,
group,
unlink-early,
unlink-late,
append
See also: OPEN, GOPEN

EXEC:<command-line>

Forks a sub process that establishes communication with its parent process
and invokes the specified program with
execvp()
.
<command-line> is a simple command
with arguments separated by single spaces. If the program name
contains a '/', the part after the last '/' is taken as ARGV[0]. If the
program name is a relative
path, the
execvp()
semantics for finding the program via
$PATH
apply. After successful program start, socat writes data to stdin of the
process and reads from its stdout using a UNIX domain socket generated by
socketpair()
per default. (example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options:
path,
fdin,
fdout,
chroot,
su,
su-d,
nofork,
pty,
stderr,
ctty,
setsid,
pipes,
login,
sigint,
sigquit
See also: SYSTEM

(Generic open) This address type tries to handle any file system entry
except directories usefully. <filename> may be a
relative or absolute path. If it already exists, its type is checked.
In case of a UNIX domain socket, socat connects; if connecting fails,
socat assumes a datagram socket and uses
sendto()
calls.
If the entry is not a socket, socat opens it applying the
O_APPEND
flag.
If it does not exist, it is opened with flag
O_CREAT
as a regular file (example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also:
OPEN,
CREATE,
UNIX-CONNECT

Communicates with a network connected on an interface using raw packets
including link level data. <interface> is the name of
the network interface. Currently only available on Linux.
Option groups: FD,SOCKET
Useful options:
pf,
type
See also: ip-recv

Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version
4 or 6 is used. It receives one packet from an unspecified peer and may send one or more answer packets to that peer.
This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process.
This allows a behaviour similar to typical UDP based servers like ntpd or
named.
Please note that the reply packets might be fetched as incoming traffic when
sender and receiver IP address are identical because there is no port number
to distinguish the sockets.
This address works well with IP-SENDTO address peers (see above).
Protocol 255 uses the raw socket with the IP header being part of the
data.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options:
pf,
fork,
range,
ttl,
broadcast
See also:
IP4-RECVFROM,
IP6-RECVFROM,
IP-SENDTO,
IP-RECV,
UDP-RECVFROM,
UNIX-RECVFROM

If <filename> already exists, it is opened.
If it does not exist, a named pipe is created and opened. Beginning with
socat version 1.4.3, the named pipe is removed when the address is closed
(but see option unlink-close
Note: When a pipe is used for both reading and writing, it works
as echo service.
Note: When a pipe is used for both reading and writing, and socat tries
to write more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat
might block. Consider using socat option, e.g.,
-b 2048
Option groups: FD,NAMED,OPEN
Useful options:
rdonly,
nonblock,
group,
user,
mode,
unlink-early
See also: unnamed pipe

PIPE

Creates an unnamed pipe and uses it for reading and writing. It works as an
echo, because everything written
to it appeares immediately as read data.
Note: When socat tries to write more bytes than the pipe can queue (Linux
2.4: 2048 bytes), socat might block. Consider, e.g., using
option
-b 2048
Option groups: FD
See also: named pipe

PROXY:<proxy>:<hostname>:<port>

Connects to an HTTP proxy server on port 8080 using TCP/IP version 4 or 6
depending on address specification, name resolution, or option
pf, and sends a CONNECT
request for hostname:port. If the proxy grants access and succeeds to
connect to the target, data transfer between socat and the target can
start. Note that the traffic need not be HTTP but can be an arbitrary
protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options:
proxyport,
ignorecr,
proxyauth,
resolve,
crnl,
bind,
connect-timeout,
mss,
sourceport,
retry
See also: SOCKS, TCP

Uses GNU readline and history on stdio to allow editing and reusing input
lines (example). This requires the GNU readline and
history libraries. Note that stdio should be a (pseudo) terminal device,
otherwise readline does not seem to work.
Option groups: FD,READLINE,TERMIOS
Useful options:
history,
noecho
See also:
STDIO

Creates a stream socket using the first and second given socket parameters
and SOCK_STREAM (see man socket\(2)) and connects to the remote-address.
The two socket parameters have to be specified by int
numbers. Consult your OS documentation and include files to find the
appropriate values. The remote-address must be the data
representation of a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option
-g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options:
bind,
setsockopt-int,
setsockopt-bin,
setsockopt-string
See also:
TCP,
UDP-CONNECT,
UNIX-CONNECT,
SOCKET-LISTEN,
SOCKET-SENDTO

SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>

Creates a datagram socket using the first three given socket parameters (see
man socket\(2)) and sends outgoing data to the remote-address. The three
socket parameters have to be specified by int
numbers. Consult your OS documentation and include files to find the
appropriate values. The remote-address must be the data
representation of a sockaddr structure without sa_family and (BSD) sa_len
components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option
-g.
Option groups: FD,SOCKET,RANGE
Useful options:
bind,
range,
setsockopt-int,
setsockopt-bin,
setsockopt-string
See also:
UDP-DATAGRAM,
IP-DATAGRAM,
SOCKET-SENDTO,
SOCKET-RECV,
SOCKET-RECVFROM

SOCKET-LISTEN:<domain>:<protocol>:<local-address>

Creates a stream socket using the first and second given socket parameters
and SOCK_STREAM (see man socket\(2)) and waits for incoming connections
on local-address. The two socket parameters have to be specified by
int numbers. Consult your OS documentation and include files
to find the appropriate values. The local-address must be the
data representation of a sockaddr structure without
sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also
use options of higher level protocols when you apply socat option
-g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options:
setsockopt-int,
setsockopt-bin,
setsockopt-string
See also:
TCP,
UDP-CONNECT,
UNIX-CONNECT,
SOCKET-LISTEN,
SOCKET-SENDTO,
SOCKET-SENDTO

Forks a sub process that establishes communication with its parent process
and invokes the specified program with
system()
. Please note that
<shell-command> [string] must
not contain ',' or "!!", and that shell meta characters may have to be
protected.
After successful program start, socat writes data to stdin of the
process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options:
path,
fdin,
fdout,
chroot,
su,
su-d,
nofork,
pty,
stderr,
ctty,
setsid,
pipes,
sigint,
sigquit
See also: EXEC

Creates a Linux TUN/TAP device and optionally assignes it the address and
netmask given by the parameters. The resulting network interface is almost
ready for use by other processes; socat serves its "wire side". This address
requires read and write access to the tunnel cloning device, usually
/dev/net/tun
, as well as permission to set some ioctl()s.
Option iff-up is required to immediately activate the interface!
Option groups: FD,NAMED,OPEN,TUN
Useful options:
iff-up,
tun-device,
tun-name,
tun-type,
iff-no-pi
See also:
ip-recv

UDP:<host>:<port>

Connects to <port> [UDP service] on
<host> [IP address] using UDP/IP version 4 or 6
depending on address specification, name resolution, or option
pf.
Please note that,
due to UDP protocol properties, no real connection is established; data has
to be sent for `connecting' to the server, and no end-of-file condition can
be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options:
ttl,
tos,
bind,
sourceport,
pf
See also:
UDP4,
UDP6,
UDP-LISTEN,
TCP,
IP

UDP4:<host>:<port>

Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4

UDP6:<host>:<port>

Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6

Waits for a UDP/IP packet arriving on <port>
[UDP service] and `connects' back to sender.
The accepted IP version is 4 or the one specified with option
pf.
Please note that,
due to UDP protocol properties, no real connection is established; data has
to arrive from the peer first, and no end-of-file condition can be
transported. Note that opening
this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options:
fork,
bind,
range,
pf
See also:
UDP,
UDP4-LISTEN,
UDP6-LISTEN,
TCP-LISTEN

Connects to <filename> assuming it is a UNIX domain
socket.
If <filename> does not exist, this is an error;
if <filename> is not a UNIX domain socket, this is an error;
if <filename> is a UNIX domain socket, but no process is listening, this is
an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX)
Useful options:
bind
See also:
UNIX-LISTEN,
UNIX-SENDTO,
TCP

Communicates with the specified peer socket, defined by [<filename>] assuming it is a UNIX domain datagram socket.
It sends packets to and receives packets from that peer socket only.
Please note that it might be neccessary to bind the
local socket to an address (e.g. /tmp/sock1, which must not exist
before).
This address type works well with socat UNIX-RECVFROM and UNIX-RECV address
peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options:
bind
See also:
UNIX-RECVFROM,
UNIX-RECV,
UNIX-CONNECT,
UDP-SENDTO,
IP-SENDTO

UNIX-RECVFROM:<filename>

Creates a UNIX domain datagram socket [<filename>].
Receives one packet and may send one or more answer packets to that peer.
This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process.
This address works well with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
Useful options:
fork
See also:
UNIX-SENDTO,
UNIX-RECV,
UNIX-LISTEN,
UDP-RECVFROM,
IP-RECVFROM

Communicates with the specified peer socket, defined by
[<filename>] assuming it is a UNIX domain socket.
It first tries to connect and, if that fails, assumes it is a datagram
socket, thus supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options:
bind
See also:
UNIX-CONNECT,
UNIX-SENDTO,
GOPEN

ABSTRACT-CONNECT:<string>

ABSTRACT-LISTEN:<string>

ABSTRACT-SENDTO:<string>

ABSTRACT-RECVFROM:<string>

ABSTRACT-RECV:<string>

ABSTRACT-CLIENT:<string>

The ABSTRACT addresses are almost identical to the related UNIX addresses
except that they do not address file system based sockets but an alternate
UNIX domain address space. To archieve this the socket address strings are
prefixed with "\0" internally. This feature is available (only?) on Linux.
Option groups are the same as with the related UNIX addresses, except that
the ABSTRACT addresses are not member of the NAMED group.

ADDRESS OPTIONS

Address options can be applied to address specifications to influence the
process of opening the addresses and the
properties of the resulting data channels.

For technical reasons not every option can be
applied to every address type; e.g., applying a socket option to a regular file
will fail. To catch most useless combinations as early as in the open phase,
the concept of option groups was introduced. Each option belongs to one
or more option groups. Options can be used only with address types that support
at least one of their option groups (but see option -g).

Address options have data types that their values must conform to.
Every address option consists of just a keyword or a keyword followed by
"=value", where value must conform to the options type.
Some address options manipulate parameters of system calls;
e.g., option sync sets the
O_SYNC
flag with the
open()
call.
Other options cause a system or library call; e.g., with option `ttl=value'
the
setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int))
call is applied.
Other
options set internal socat variables that are used during data transfer;
e.g., `crnl' causes explicit character conversions.
A few options have more complex implementations; e.g., su-d
(substuser-delayed) inquires some user and group infos, stores them, and
applies them later after a possible
chroot()
call.

If multiple options are given to an address, their sequence in the address specification has (almost) no
effect on the sequence of their execution/application. Instead, socat has
built in an option phase model that tries to bring the options in a useful
order. Some options exist in different forms (e.g.,
unlink, unlink-early, unlink-late) to control the time of their execution.

If the same option is specified more than once within one address
specification, with equal or different values, the effect depends on the kind of option. Options
resulting in function calls like
setsockopt()
cause multiple
invocations. With options that set parameters for a required call like
open()
or set internal flags, the value of the last option occurrence is effective.

The existence or semantics of many options are system dependent. Socat
usually does NOT try to emulate missing libc or kernel features, it just
provides an
interface to the underlying system. So, if an operating system lacks a feature,
the related option is simply not available on this platform.

The following paragraphs introduce just the more common address options. For
a more comprehensive reference and to find information about canonical option
names, alias names, option phases, and platforms see file xio.help.

FD option group

This option group contains options that are applied to a UN*X
style file descriptor, no matter how it was generated.
Because all current socat address types are file descriptor based, these
options may be applied to any address.
Note: Some of these options are also member of another option group, that
provides an other, non-fd based mechanism.
For these options, it depends on the actual address type and its option groups
which mechanism is used. The second, non-fd based mechanism is prioritized.

cloexec=<bool>

Sets the
FD_CLOEXEC
flag with the
fcntl()
system call to value
<bool>. If set,
the file descriptor is closed on
exec()
family function calls. Socat
internally handles
this flag for the fds it controls, so in most cases there will be no need to
apply this option.

setlk

Tries to set a discretionary write lock to the whole file using the
fcntl(fd,
F_SETLK, ...)
system call. If the file is already locked, this call results
in an error.
On Linux, when the file permissions for group are "S" (g-x,g+s), and the
file system is locally mounted with the "mand" option, the lock is
mandatory, i.e. prevents other processes from opening the file.

setlkw

Tries to set a discretionary waiting write lock to the whole file using the
fcntl(fd, F_SETLKW, ...)
system call. If the file is already locked,
this call blocks.
See option setlk for information about making this
lock mandatory.

setlk-rd

Tries to set a discretionary read lock to the whole file using the
fcntl(fd,
F_SETLK, ...)
system call. If the file is already write locked, this call
results in an error.
See option setlk for information about making this
lock mandatory.

setlkw-rd

Tries to set a discretionary waiting read lock to the whole file using the
fcntl(fd, F_SETLKW, ...)
system call. If the file is already write
locked, this call blocks.
See option setlk for information about making this
lock mandatory.

flock-ex

Tries to set a blocking exclusive advisory lock to the file using the
flock(fd, LOCK_EX)
system call. Socat hangs in this call if the file
is locked by another process.

flock-ex-nb

Tries to set a nonblocking exclusive advisory lock to the file using the
flock(fd, LOCK_EX|LOCK_NB)
system call. If the file is already locked,
this option results in an error.

flock-sh

Tries to set a blocking shared advisory lock to the file using the
flock(fd, LOCK_SH)
system call. Socat hangs in this call if the file
is locked by another process.

flock-sh-nb

Tries to set a nonblocking shared advisory lock to the file using the
flock(fd, LOCK_SH|LOCK_NB)
system call. If the file is already locked,
this option results in an error.

lock

Sets a blocking lock on the file. Uses the setlk or flock mechanism
depending on availability on the particular platform. If both are available,
the POSIX variant (setlkw) is used.

user=<user>

Sets the <user> (owner) of the stream.
If the address is member of the NAMED option group,
socat uses the
chown()
system call after opening the
file or binding to the UNIX domain socket (race condition!).
Without filesystem entry, socat sets the user of the stream
using the
fchown()
system call.
These calls might require root privilege.

user-late=<user>

Sets the owner of the fd to <user> with the
fchown()
system call after opening
or connecting the channel.
This is useful only on file system entries.

group=<group>

Sets the <group> of the stream.
If the address is member of the NAMED option group,
socat uses the
chown()
system call after opening the
file or binding to the UNIX domain socket (race condition!).
Without filesystem entry, socat sets the group of the stream
with the
fchown()
system call.
These calls might require group membership or root privilege.

group-late=<group>

Sets the group of the fd to <group> with the
fchown()
system call after opening
or connecting the channel.
This is useful only on file system entries.

mode=<mode>

Sets the <mode> [mode_t] (permissions) of the stream.
If the address is member of the NAMED option group and
uses the
open()
or
creat()
call, the mode is applied with these.
If the address is member of the NAMED option group without using these
system calls, socat uses the
chmod()
system call after opening the
filesystem entry or binding to the UNIX domain socket (race condition!).
Otherwise, socat sets the mode of the stream
using
fchmod()
.
These calls might require ownership or root privilege.

perm-late=<mode>

Sets the permissions of the fd to value <mode>
[mode_t] using the
fchmod()
system call after
opening or connecting the channel.
This is useful only on file system entries.

append=<bool>

Always writes data to the actual end of file.
If the address is member of the OPEN option group,
socat uses the
O_APPEND
flag with the
open()
system call
(example).
Otherwise, socat applies the
fcntl(fd, F_SETFL, O_APPEND)
call.

nonblock=<bool>

Tries to open or use file in nonblocking mode. Its only effects are that the
connect()
call of TCP addresses does not block, and that opening a
named pipe for reading does not block.
If the address is member of the OPEN option group,
socat uses the
O_NONBLOCK
flag with the
open()
system call.
Otherwise, socat applies the
fcntl(fd, F_SETFL, O_NONBLOCK)
call.

binary

Opens the file in binary mode to avoid implicit line terminator
conversions (Cygwin).

text

Opens the file in text mode to force implicit line terminator conversions
(Cygwin).

noinherit

Does not keep this file open in a spawned process (Cygwin).

cool-write

Takes it easy when write fails with EPIPE or ECONNRESET and logs the message
with notice level instead of error.
This prevents the log file from being filled with useless error messages
when socat is used as a high volume server or proxy where clients often
abort the connection.
This option is experimental.

end-close

Changes the (address dependent) method of ending a connection to just close
the file descriptors. This is useful when the connection is to be reused by
or shared with other processes (example).
Normally, socket connections will be ended with shutdown(2) which
terminates the socket even if it is shared by multiple processes.
close(2) "unlinks" the socket from the process but keeps it active as
long as there are still links from other processes.
Similarly, when an address of type EXEC or SYSTEM is ended, socat usually
will explicitely kill the sub process. With this option, it will just close
the file descriptors.

shut-none

Changes the (address dependent) method of shutting down the write part of a
connection to not do anything.

shut-down

Changes the (address dependent) method of shutting down the write part of a
connection to shutdown\(fd, SHUT_WR). Is only useful with sockets.

shut-close

Changes the (address dependent) method of shutting down the write part of a
connection to close\(fd).

shut-null

When one address indicates EOF, socat will send a zero sized packet to the
write channel of the other address to transfer the EOF condition. This is
useful with UDP and other datagram protocols. Has been tested against
netcat and socat with option null-eof.

Applies the
lseek(fd, <offset>, SEEK_CUR)
(or
lseek64
) system
call, thus positioning the file pointer <offset> [off_t or
off64_t] bytes relatively to its current position (which
is usually 0). Please note that a missing value defaults to 1, not 0.

Applies the
ftruncate(fd, <offset>)
(or
ftruncate64
if available) system call, thus
truncating the file at the position <offset> [off_t or
off64_t]. Please note that a missing value defaults to 1,
not 0.

secrm=<bool>

unrm=<bool>

compr=<bool>

ext2-sync=<bool>

immutable=<bool>

ext2-append=<bool>

nodump=<bool>

ext2-noatime=<bool>

journal-data=<bool>

notail=<bool>

dirsync=<bool>

These options change non standard file attributes on operating systems and
file systems that support these features, like Linux with ext2fs,
ext3fs, or reiserfs. See man 1 chattr for information on these options.
Please note that there might be a race condition between creating the file
and applying these options.

PROCESS option group

Options of this group change the process properties instead of just affecting
one data channel.
For EXEC and SYSTEM addresses and for LISTEN and CONNECT type addresses with
option FORK,
these options apply to the child processes instead of the main socat process.

Changes the <user> (owner) of the process after processing
the address. This call might require root privilege. Please note that this
option does not drop group related privileges. Check if option
su better fits your needs.

Changes the <user> (owner) and groups of the process after
processing the address (example). This call might require root privilege.

su-d=<user>

Short name for substuser-delayed.
Changes the <user>
(owner) and groups of the process after processing the address (example).
The user and his groups are retrieved before a possible
chroot()
. This call might require root privilege.

setpgid=<pid_t>

Makes the process a member of the specified process group
<pid_t>. If no value
is given, or if the value is 0 or 1, the process becomes leader of a new
process group.

Since version 1.4.0, socat per default tries to determine a prompt -
that is then passed to the readline call - by remembering the last
incomplete line of the output. With this option, socat does not pass a
prompt to readline, so it begins line editing in the first column
of the terminal.

noecho=<pattern>

Specifies a regular pattern for a prompt that prevents the following input
line from being displayed on the screen and from being added to the history.
The prompt is defined as the text that was output to the readline address
after the lastest newline character and before an input character was
typed. The pattern is a regular expression, e.g.
"^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex\(7) for details.
(example)

prompt=<string>

Passes the string as prompt to the readline function. readline prints this
prompt when stepping through the history. If this string matches a constant
prompt issued by an interactive program on the other socat address,
consistent look and feel can be archieved.

APPLICATION option group

This group contains options that work at data level.
Note that these options only apply to the "raw" data transferred by socat,
but not to protocol data used by addresses like
PROXY.

cr

Converts the default line termination character NL ('\n', 0x0a) to/from CR
('\r', 0x0d) when writing/reading on this channel.

When EOF occurs on this channel, socat ignores it and tries to read more
data (like "tail -f") (example).

readbytes=<bytes>

socat reads only so many bytes from this address (the address provides
only so many bytes for transfer and pretends to be at EOF afterwards).
Must be greater than 0.

lockfile=<filename>

If lockfile exists, exits with error. If lockfile does not exist, creates it
and continues, unlinks lockfile on exit.

waitlock=<filename>

If lockfile exists, waits until it disappears. When lockfile does not exist,
creates it and continues, unlinks lockfile on exit.

escape=<int>

Specifies the numeric code of a character that triggers EOF on the input
stream. It is useful with a terminal in raw mode
(example).

SOCKET option group

These options are intended for all kinds of sockets, e.g. IP or UNIX domain. Most are applied with a
setsockopt()
call.

bind=<sockname>

Binds the socket to the given socket address using the
bind()
system
call. The form of <sockname> is socket domain dependent:
IP4 and IP6 allow the form [hostname|hostaddress][:(service|port)] (example),
UNIX domain sockets require <filename>.

connect-timeout=<seconds>

Abort the connection attempt after <seconds> [timeval]
with error status.

so-bindtodevice=<interface>

Binds the socket to the given <interface>.
This option might require root privilege.

Forces the use of the specified IP version or protocol. <string> can be
something like "ip4" or "ip6". The resulting value is
used as first argument to the
socket()
or
socketpair()
calls.
This option affects address resolution and the required syntax of bind and
range options.

type=<type>

Sets the type of the socket, specified as second argument to the
socket()
or
socketpair()
calls, to <type>
[int]. Address resolution is not affected by this option.
Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3
means raw socket.

prototype

Sets the protocol of the socket, specified as third argument to the
socket()
or
socketpair()
calls, to <prototype>
[int]. Address resolution is not affected by this option.
6 means TCP, 17 means UDP.

Invokes setsockopt() for the socket with the given parameters. level
[int] is used as second argument to setsockopt() and
specifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or SOL_SOCKET for
the socket layer (1 on Linux). optname [int] is the
third argument to setsockopt() and tells which socket option is to be
set. For the actual numbers you might have to look up the appropriate include
files of your system. The 4th setsockopt() parameter, value
[int], is passed to the function per pointer, and for the
length parameter sizeof\(int) is taken implicitely.

setsockopt-bin=<level>:<optname>:<optval>

Like setsockopt-int, but <optval> must be provided in
dalan format and specifies an arbitrary sequence of bytes;
the length parameter is automatically derived from the data.

setsockopt-string=<level>:<optname>:<optval>

Like setsockopt-int, but <optval> must be a string.
This string is passed to the function with trailing null character, and the
length parameter is automatically derived from the data.

UNIX option group

These options apply to UNIX domain based addresses.

unix-tightsocklen=[0|1]

On socket operations, pass a socket address length that does not include the
whole
struct sockaddr_un
record but (besides other components) only
the relevant part of the filename or abstract string. Default is 1.

IP4 and IP6 option groups

These options can be used with IPv4 and IPv6 based sockets.

tos=<tos>

Sets the TOS (type of service) field of outgoing packets to <tos>
[byte] (see RFC 791).

ttl=<ttl>

Sets the TTL (time to live) field of outgoing packets to <ttl>
[byte].

ip-options=<data>

Sets IP options like source routing. Must be given in binary form,
recommended format is a leading "x" followed by an even number of hex
digits. This option may be used multiple times, data are appended.
E.g., to connect to host 10.0.0.1 via some gateway using a loose source
route, use the gateway as address parameter and set a loose source route
using the option
ip-options=x8307040a000001
.
IP options are defined in RFC 791.

mtudiscover=<0|1|2>

Takes 0, 1, 2 to never, want, or always use path MTU discover on this
socket.

Makes the socket member of the specified multicast group. This is currently
only implemented for IPv4. The option takes the IP address of the multicast
group and info about the desired network interface. The most common syntax
is the first one, while the others are only available on systems that
provide struct mreqn (Linux).
The indices of active network interfaces can be shown using the utility
procan.

ip-multicast-if=<hostname>

Specifies hostname or address of the network interface to be used for
multicast traffic.

ip-multicast-loop=<bool>

Specifies if outgoing multicast traffic should loop back to the interface.

ip-multicast-ttl=<byte>

Sets the TTL used for outgoing multicast traffic. Default is 1.

res-debug

res-aaonly

res-usevc

res-primary

res-igntc

res-recurse

res-defnames

res-stayopen

res-dnsrch

These options set the corresponding resolver (name resolution) option flags.
Append "=0" to clear a default option. See man resolver\(5) for more
information on these options. Note: these options are valid only for the
address they are applied to.

IP6 option group

These options can only be used on IPv6 based sockets. See IP
options for options that can be applied to both IPv4 and IPv6
sockets.

ipv6only=<bool>

Sets the IPV6_V6ONLY socket option. If 0, the TCP stack will also accept
connections using IPv4 protocol on the same port. The default is system
dependent.

Enables the time stamp option that allows RTT recalculation on existing
connections (Tru64).

SCTP option group

These options may be applied to SCTP stream sockets.

sctp-nodelay

Sets the SCTP_NODELAY socket option that disables the Nagle algorithm.

sctp-maxseg=<bytes>

Sets the SCTP_MAXSEG socket option to <bytes> [int]. This
value is then proposed to the peer with the SYN or SYN/ACK packet.

UDP, TCP, and SCTP option groups

Here we find options that are related to the network port mechanism and thus
can be used with UDP, TCP, and SCTP client and server addresses.

sourceport=<port>

For outgoing (client) TCP and UDP connections, it sets the source
<port> using an extra
bind()
call.
With TCP or UDP listen addresses, socat immediately shuts down the
connection if the client does not use this sourceport (example).

lowport

Outgoing (client) TCP and UDP connections with this option use
an unused random source port between 640 and 1023 incl. On UNIX class operating
systems, this requires root privilege, and thus indicates that the
client process is authorized by local root.
TCP and UDP listen addresses with this option immediately shut down the
connection if the client does not use a sourceport <= 1023.
This mechanism can provide limited authorization under some circumstances.

SOCKS option group

When using SOCKS type addresses, some socks specific options can be set.

socksport=<tcp service>

Overrides the default "socks" service or port 1080 for the socks server
port with <TCP service>.

socksuser=<user>

Sends the <user> [string] in the username field to the
socks server. Default is the actual user name ($LOGNAME or $USER) (example).

HTTP option group

Options that can be provided with HTTP type addresses. The only HTTP address
currently implemented is proxy-connect.

The HTTP protocol requires the use of CR+NL as line terminator. When a proxy
server violates this standard, socat might not understand its answer.
This option directs socat to interprete NL as line terminator and
to ignore CR in the answer. Nevertheless, socat sends CR+NL to the proxy.

proxyauth=<username>:<password>

Provide "basic" authentication to the proxy server. The argument to the
option is used with a "Proxy-Authorization: Base" header in base64 encoded
form.
Note: username and password are visible for every user on the local machine
in the process list; username and password are transferred to the proxy
server unencrypted (base64 encoded) and might be sniffed.

resolve

Per default, socat sends to the proxy a CONNECT request containing the
target hostname. With this option, socat resolves the hostname locally and
sends the IP address. Please note that, according to RFC 2396, only name
resolution to IPv4 addresses is implemented.

RANGE option group

These options check if a connecting client should be granted access. They can
be applied to listening and receiving network sockets. tcp-wrappers options
fall into this group.

range=<address-range>

After accepting a connection, tests if the peer is within range. For
IPv4 addresses, address-range takes the form address/bits, e.g.
10.0.0.0/8, or address:mask, e.g. 10.0.0.0:255.0.0.0 (example); for IPv6, it is [ip6-address/bits], e.g. [::1/128].
If the client address does not match, socat issues a warning and keeps
listening/receiving.

tcpwrap[=<name>]

Uses Wietse Venema's libwrap (tcpd) library to determine
if the client is allowed to connect. The configuration files are
/etc/hosts.allow and /etc/hosts.deny per default, see "man 5 hosts_access"
for more information. The optional <name> (type string)
is passed to the wrapper functions as daemon process name (example).
If omitted, the basename of socats invocation (argv[0]) is passed.
If both tcpwrap and range options are applied to an address, both
conditions must be fulfilled to allow the connection.

allow-table=<filename>

Takes the specified file instead of /etc/hosts.allow.

deny-table=<filename>

Takes the specified file instead of /etc/hosts.deny.

tcpwrap-etc=<directoryname>

Looks for hosts.allow and hosts.deny in the specified directory. Is
overridden by options hosts-allow
and hosts-deny.

LISTEN option group

Options specific to listening sockets.

backlog=<count>

Sets the backlog value passed with the
listen()
system call to <count>
[int]. Default is 5.

max-children=<count>

Limits the number of concurrent child processes [int].
Default is no limit.

CHILD option group

Options for addresses with multiple connections via child processes.

fork

After establishing a connection, handles its channel in a child process and
keeps the parent process attempting to produce more connections, either by
listening or by connecting in a loop (example).
SSL-CONNECT and SSL-LISTEN differ in when they actually fork off the child:
SSL-LISTEN forks before the SSL handshake, while SSL-CONNECT forks
afterwards.
RETRY and FOREVER options are not inherited by the child process.
On some operating systems (e.g. FreeBSD) this option does not work for
UDP-LISTEN addresses.

EXEC option group

Options for addresses that invoke a program.

path=<string>

Overrides the PATH environment variable for searching the program with
<string>. This
$PATH
value is effective in the child process too.

login

Prefixes
argv[0]
for the
execvp()
call with '-', thus making a
shell behave as login shell.

FORK option group

EXEC or SYSTEM addresses invoke a program using a child process and transfer data between socat and the program. The interprocess communication mechanism can be influenced with the following options. Per
default, a
socketpair()
is created and assigned to stdin and stdout of
the child process, while stderr is inherited from the socat process, and the
child process uses file descriptors 0 and 1 for communicating with the main
socat process.

nofork

Does not fork a subprocess for executing the program, instead calls execvp\()
or system\() directly from the actual socat instance. This avoids the
overhead of another process between the program and its peer,
but introduces a lot of restrictions:

for the second address (the one with option nofork), options
append, cloexec, flock, user, group, mode, nonblock,
perm-late, setlk, and setpgid cannot be applied. Some of these could be
used on the first address though.

pipes

Creates a pair of unnamed pipes for interprocess communication instead of a
socket pair.

openpty

Establishes communication with the sub process using a pseudo terminal
created with
openpty()
instead of the default (socketpair or ptmx).

ptmx

Establishes communication with the sub process using a pseudo terminal
created by opening /dev/ptmx or /dev/ptc instead of the default
(socketpair).

pty

Establishes communication with the sub process using a pseudo terminal
instead of a socket pair. Creates the pty with an available mechanism. If
openpty and ptmx are both available, it uses ptmx because this is POSIX
compliant (example).

Directs stderr of the sub process to its output channel by making stderr a
dup()
of stdout (example).

fdin=<fdnum>

Assigns the sub processes input channel to its file descriptor
<fdnum>
instead of stdin (0). The program started from the subprocess has to use
this fd for reading data from socat (example).

fdout=<fdnum>

Assigns the sub processes output channel to its file descriptor
<fdnum>
instead of stdout (1). The program started from the subprocess has to use
this fd for writing data to socat (example).

sighup, sigint, sigquit

Has socat pass signals of this type to the sub process.
If no address has this option, socat terminates on these signals.

TERMIOS option group

For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the terminal parameters defined in the UN*X termios mechanism are made available as address option parameters.
Please note that changes of the parameters of your interactive terminal
remain effective after socat's termination, so you might have to enter "reset"
or "stty sane" in your shell afterwards.
For EXEC and SYSTEM addresses with option PTY,
these options apply to the pty by the child processes.

b0

Disconnects the terminal.

b19200

Sets the serial line speed to 19200 baud. Some other rates are possible; use
something like socat -hh |grep ' b[1-9]' to find all speeds supported by
your implementation.
Note: On some operating systems, these options may not be
available. Use ispeed or ospeed
instead.

echo=<bool>

Enables or disables local echo.

icanon=<bool>

Sets or clears canonical mode, enabling line buffering and some special
characters.

raw

Sets raw mode, thus passing input and output almost unprocessed. This option is obsolete, use option rawer or cfmakeraw instead.

Sets raw mode by invoking cfmakeraw() or by simulating this call. This option implicitly turns off echo.

ignbrk=<bool>

Ignores or interpretes the BREAK character (e.g., ^C)

brkint=<bool>

bs0

bs1

bsdly=<0|1>

clocal=<bool>

cr0cr1cr2cr3

Sets the carriage return delay to 0, 1, 2, or 3, respectively.
0 means no delay, the other values are terminal dependent.

crdly=<0|1|2|3>

cread=<bool>

crtscts=<bool>

cs5cs6cs7cs8

Sets the character size to 5, 6, 7, or 8 bits, respectively.

csize=<0|1|2|3>

cstopb=<bool>

Sets two stop bits, rather than one.

dsusp=<byte>

Sets the value for the VDSUSP character that suspends the current foreground
process and reactivates the shell (all except Linux).

echoctl=<bool>

Echos control characters in hat notation (e.g. ^A)

echoe=<bool>

echok=<bool>

echoke=<bool>

echonl=<bool>

echoprt=<bool>

eof=<byte>

eol=<byte>

eol2=<byte>

erase=<byte>

discard=<byte>

ff0

ff1

ffdly=<bool>

flusho=<bool>

hupcl=<bool>

icrnl=<bool>

iexten=<bool>

igncr=<bool>

ignpar=<bool>

imaxbel=<bool>

inlcr=<bool>

inpck=<bool>

intr=<byte>

isig=<bool>

ispeed=<unsigned-int>

Set the baud rate for incoming data on this line.
See also: ospeed, b19200

istrip=<bool>

iuclc=<bool>

ixany=<bool>

ixoff=<bool>

ixon=<bool>

kill=<byte>

lnext=<byte>

min=<byte>

nl0

Sets the newline delay to 0.

nl1

nldly=<bool>

noflsh=<bool>

ocrnl=<bool>

ofdel=<bool>

ofill=<bool>

olcuc=<bool>

onlcr=<bool>

onlret=<bool>

onocr=<bool>

opost=<bool>

Enables or disables output processing; e.g., converts NL to CR-NL.

ospeed=<unsigned-int>

Set the baud rate for outgoing data on this line.
See also: ispeed, b19200

parenb=<bool>

Enable parity generation on output and parity checking for input.

parmrk=<bool>

parodd=<bool>

pendin=<bool>

quit=<byte>

reprint=<byte>

sane

Brings the terminal to something like a useful default state.

start=<byte>

stop=<byte>

susp=<byte>

swtc=<byte>

tab0

tab1

tab2

tab3

tabdly=<unsigned-int>

time=<byte>

tostop=<bool>

vt0

vt1

vtdly=<bool>

werase=<byte>

xcase=<bool>

xtabs

i-pop-all

With UNIX System V STREAMS, removes all drivers from the stack.

i-push=<string>

With UNIX System V STREAMS, pushes the driver (module) with the given name
(string) onto the stack. For example, to make sure that a
character device on Solaris supports termios etc, use the following options:
i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat

Generates a symbolic link that points to the actual pseudo terminal
(pty). This might help
to solve the problem that ptys are generated with more or less
unpredictable names, making it difficult to directly access the socat
generated pty automatically. With this option, the user can specify a "fix"
point in the file hierarchy that helps him to access the actual pty
(example).
Beginning with socat version 1.4.3, the symbolic link is removed when
the address is closed (but see option unlink-close).

wait-slave

Blocks the open phase until a process opens the slave side of the pty.
Usually, socat continues after generating the pty with opening the next
address or with entering the transfer loop. With the wait-slave option,
socat waits until some process opens the slave side of the pty before
continuing.
This option only works if the operating system provides the poll()
system call. And it depends on an undocumented behaviour of pty's, so it
does not work on all operating systems. It has successfully been tested on
Linux, FreeBSD, NetBSD, and on Tru64 with openpty.

pty-interval=<seconds>

When the wait-slave option is set, socat
periodically checks the HUP condition using poll() to find if the pty's
slave side has been opened. The default polling interval is 1s. Use the
pty-interval option [timeval] to change this value.

Selects the list of ciphers that may be used for the connection.
See the man page of
ciphers
, section CIPHER LIST FORMAT, for
detailed information about syntax, values, and default of <cipherlist>.
Several cipher strings may be given, separated by ':'.
Some simple cipher strings:

3DES

Uses a cipher suite with triple DES.

MD5

Uses a cipher suite with MD5.

aNULL

Uses a cipher suite without authentication.

NULL

Does not use encryption.

HIGH

Uses a cipher suite with "high" encryption.

Note that the peer must support the selected property, or the negotiation
will fail.

method=<ssl-method>

Sets the protocol version to be used. Valid strings (not case sensitive)
are:

SSL2

Select SSL protocol version 2.

SSL3

Select SSL protocol version 3.

SSL23

Select the best available SSL or TLS protocol. This is the default when
this option is not provided.

TLS1

Select TLS protocol version 1.

TLS1.1

Select TLS protocol version 1.1.

TLS1.2

Select TLS protocol version 1.2.

DTLS1

Select DTLS protocol version 1.

verify=<bool>

Controls check of the peer's certificate. Default is 1 (true). Disabling
verify might open your socket for everyone, making the encryption useless!

cert=<filename>

Specifies the file with the certificate and private key for authentication.
The certificate must be in OpenSSL format (*.pem).
With openssl-listen, use of this option is strongly
recommended. Except with cipher aNULL, "no shared ciphers" error will
occur when no certificate is given.

key=<filename>

Specifies the file with the private key. The private key may be in this
file or in the file given with the cert option. The party that has
to proof that it is the owner of a certificate needs the private key.

dhparams=<filename>

Specifies the file with the Diffie Hellman parameters. These parameters may
also be in the file given with the cert
option in which case the dhparams option is not needed.

cafile=<filename>

Specifies the file with the trusted (root) authority certificates. The file
must be in PEM format and should contain one or more certificates. The party
that checks the authentication of its peer trusts only certificates that are
in this file.

capath=<dirname>

Specifies the directory with the trusted (root) certificates. The directory
must contain certificates in PEM format and their hashes (see OpenSSL
documentation)

egd=<filename>

On some systems, openssl requires an explicit source of random data. Specify
the socket name where an entropy gathering daemon like egd provides random
data, e.g. /dev/egd-pool.

pseudo

On systems where openssl cannot find an entropy source and where no entropy
gathering daemon can be utilized, this option activates a mechanism for
providing pseudo entropy. This is archieved by taking the current time in
microseconds for feeding the libc pseudo random number generator with an
initial value. openssl is then feeded with output from random\() calls.
NOTE:This mechanism is not sufficient for generation of secure keys!

compress

Enable or disable the use of compression for a connection. Setting this to
"none" disables compression, setting it to "auto" lets OpenSSL choose the best
available algorithm supported by both parties. The default is to not touch any
compression-related settings.
NOTE: Requires OpenSSL 0.9.8 or higher and disabling compression with
OpenSSL 0.9.8 affects all new connections in the process.

commonname=<string>

Specify the commonname that the peer certificate must match. With
OPENSSL-CONNECT address this overrides the
given hostname or IP target address; with
OPENSSL-LISTEN this turns on check of peer
certificates commonname. This option has only meaning when option
verify is not disabled and the choosen cipher
provides a peer certificate.

fips

Enables FIPS mode if compiled in. For info about the FIPS encryption
implementation standard see http://oss-institute.org/fips-faq.html.
This mode might require that the involved certificates are generated with a
FIPS enabled version of openssl. Setting or clearing this option on one
socat address affects all OpenSSL addresses of this process.

RETRY option group

Options that control retry of some system calls, especially connection
attempts.

retry=<num>

Number of retries before the connection or listen attempt is aborted.
Default is 0, which means just one attempt.

interval=<timespec>

Time between consecutive attempts (seconds,
[timespec]). Default is 1 second.

forever

Performs an unlimited number of retry attempts.

TUN option group

Options that control Linux TUN/TAP interface device addresses.

tun-device=<device-file>

Instructs socat to take another path for the TUN clone device. Default is
/dev/net/tun.

tun-name=<if-name>

Gives the resulting network interface a specific name instead of the system
generated (tun0, tun1, etc.)

tun-type=[tun|tap]

Sets the type of the TUN device; use this option to generate a TAP
device. See the Linux docu for the difference between these types.
When you try to establish a tunnel between two TUN devices, their types
should be the same.

iff-no-pi

Sets the IFF_NO_PI flag which controls if the device includes additional
packet information in the tunnel.
When you try to establish a tunnel between two TUN devices, these flags
should have the same values.

iff-up

Sets the TUN network interface status UP. Strongly recommended.

iff-broadcast

Sets the BROADCAST flag of the TUN network interface.

iff-debug

Sets the DEBUG flag of the TUN network interface.

iff-loopback

Sets the LOOPBACK flag of the TUN network interface.

iff-pointopoint

Sets the POINTOPOINT flag of the TUN device.

iff-notrailers

Sets the NOTRAILERS flag of the TUN device.

iff-running

Sets the RUNNING flag of the TUN device.

iff-noarp

Sets the NOARP flag of the TUN device.

iff-promisc

Sets the PROMISC flag of the TUN device.

iff-allmulti

Sets the ALLMULTI flag of the TUN device.

iff-master

Sets the MASTER flag of the TUN device.

iff-slave

Sets the SLAVE flag of the TUN device.

iff-multicast

Sets the MULTICAST flag of the TUN device.

iff-portsel

Sets the PORTSEL flag of the TUN device.

iff-automedia

Sets the AUTOMEDIA flag of the TUN device.

iff-dynamic

Sets the DYNAMIC flag of the TUN device.

DATA VALUES

This section explains the different data types that address parameters and
address options can take.

address-range

Is currently only implemented for IPv4 and IPv6. See address-option
`range'

If the first character is a decimal digit, the value is read with
strtoul()
as unsigned integer specifying a group id. Otherwise, it
must be an existing group name.

int

A number following the rules of the
strtol()
function with base
"0", i.e. decimal number, octal number with leading "0", or hexadecimal
number with leading "0x". The value must fit into a C int.

interface

A string specifying the device name of a network interface
as shown by ifconfig or procan, e.g. "eth0".

IP address

An IPv4 address in numbers-and-dots notation, an IPv6 address in hex
notation enclosed in brackets, or a hostname that resolves to an IPv4 or an
IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1

IPv4 address

An IPv4 address in numbers-and-dots notation or a hostname that resolves to
an IPv4 address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2

IPv6 address

An iPv6 address in hexnumbers-and-colons notation enclosed in brackets, or a
hostname that resolves to an IPv6 address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0],
ip6name.domain.org

long

A number read with
strtol()
. The value must fit into a C long.

long long

A number read with
strtoll()
. The value must fit into a C long long.

off_t

An implementation dependend signed number, usually 32 bits, read with strtol
or strtoll.

off64_t

An implementation dependend signed number, usually 64 bits, read with strtol
or strtoll.

A sequence of characters, not containing '\0' and, depending on
the position within the command line, ':', ',', or "!!". Note
that you might have to escape shell meta characters in the command line.

TCP service

A service name, not starting with a digit, that is resolved by
getservbyname()
, or an unsigned int 16 bit number read with
strtoul()
.

timeval

A double float specifying seconds; the number is mapped into a
struct timeval, consisting of seconds and microseconds.

timespec

A double float specifying seconds; the number is mapped into a
struct timespec, consisting of seconds and nanoseconds.

UDP service

A service name, not starting with a digit, that is resolved by
getservbyname()
, or an unsigned int 16 bit number read with
strtoul()
.

unsigned int

A number read with
strtoul()
. The value must fit into a C unsigned
int.

user

If the first character is a decimal digit, the value is read with
strtoul()
as unsigned integer specifying a user id. Otherwise, it must
be an existing user name.

EXAMPLES

socat - TCP4:www.domain.org:80

transfers data between STDIO (-) and a
TCP4 connection to port 80 of host
www.domain.org. This example results in an interactive connection similar to
telnet or netcat. The stdin terminal parameters are not changed, so you may
close the relay with ^D or abort it with ^C.

this is similar to the previous example, but you can edit the current line in a
bash like manner (READLINE) and use the
history file .http_history; socat prints messages about
progress (-d -d). The port is specified by service name
(www), and correct network line termination characters
(crnl) instead of NL are used.

socat TCP4-LISTEN:www TCP4:www.domain.org:www

installs a simple TCP port forwarder. With
TCP4-LISTEN it listens on local port "www" until a
connection comes in, accepts it, then connects to the remote host
(TCP4) and starts data transfer. It will not accept
a econd connection.

TCP port forwarder, each side bound to another local IP address
(bind). This example handles an almost
arbitrary number of parallel or consecutive connections by
fork'ing a new
process after each
accept()
. It provides a little security by
su'ing to user
nobody after forking; it only permits connections from the private 10 network
(range); due to reuseaddr, it
allows immediate restart after master process's termination, even if some child
sockets are not completely shut down.
With -lmlocal2, socat logs to stderr until successfully
reaching the accept loop. Further logging is directed to syslog with facility
local2.

a simple server that accepts connections
(TCP4-LISTEN) and fork's a new
child process for each connection; every child acts as single relay.
The client must match the rules for daemon process name "script" in
/etc/hosts.allow and /etc/hosts.deny, otherwise it is refused access (see "man
5 hosts_access").
For EXEC'uting the program, the child process
chroot's
to /home/sandbox, su's to user sandbox, and then starts
the program /home/sandbox/bin/myscript. Socat and
myscript communicate via a pseudo tty (pty); myscript's
stderr is redirected to stdout,
so its error messages are transferred via socat to the connected client.

mail.sh is a shell script, distributed with socat, that implements a
simple
SMTP client. It is programmed to "speak" SMTP on its FDs 3 (in) and 4 (out).
The fdin and fdout options tell socat
to use these FDs for communication with
the program. Because mail.sh inherits stdin and stdout while socat does not
use them, the script can read a
mail body from stdin. Socat makes alias1 your local source address
(bind), cares for correct network line termination
(crnl) and sends
at most 512 data bytes per packet (mss).

socat -,escape=0x0f /dev/ttyS0,rawer,crnl

opens an interactive connection via the serial line, e.g. for talking with a
modem. rawer sets the console's and
ttyS0's terminal parameters to practicable values, crnl
converts to correct newline characters. escape allows to
terminate the socat process with character control-O.
Consider using READLINE instead of the first address.

with UNIX-LISTEN, socat opens a listening
UNIX domain socket /tmp/.X11-unix/X1. This path corresponds
to local XWindow display :1 on your machine, so XWindow client connections to
DISPLAY=:1 are accepted. Socat then speaks with
the SOCKS4 server host.victim.org that might permit
sourceport 20 based connections due to an FTP related
weakness in its static IP filters. Socat
pretends to be invoked by socksuser nobody, and
requests to be connected to
loopback port 6000 (only weak sockd configurations will allow this). So we get
a connection to the victims XWindow server and, if it does not require MIT
cookies or Kerberos authentication, we can start work. Please note that there
can only be one connection at a time, because TCP can establish only one
session with a given set of addresses and ports.

socat -u /tmp/readdata,seek-end=0,ignoreeof -

this is an example for unidirectional data transfer
(-u). Socat transfers data
from file /tmp/readdata (implicit address GOPEN), starting
at its current end (seek-end=0 lets socat start
reading at current end of file; use seek=0 or no
seek option to first read the existing data) in a "tail -f" like mode
(ignoreeof). The "file"
might also be a listening UNIX domain socket (do not use a seek option then).

EXEC'utes an ssh session to server. Uses a pty for communication between socat and
ssh, makes it ssh's controlling tty (ctty),
and makes this pty the owner of
a new process group (setsid), so ssh accepts the password from socat.

implements a simple network based message collector.
For each client connecting to port 3334, a new child process is generated (option fork).
All data sent by the clients are append'ed to the file /tmp/in.log.
If the file does not exist, socat creat's it.
Option reuseaddr allows immediate restart of the server
process.

wraps a command line history (READLINE) around the EXEC'uted ftp client utility.
This allows editing and reuse of FTP commands for relatively comfortable
browsing through the ftp directory hierarchy. The password is echoed!
pty is required to have ftp issue a prompt.
Nevertheless, there may occur some confusion with the password and FTP
prompts.

generates a pseudo terminal
device (PTY) on the client that can be reached under the
symbolic link$HOME/dev/vmodem0.
An application that expects a serial line or modem
can be configured to use $HOME/dev/vmodem0; its traffic will be directed
to a modemserver via ssh where another socat instance links it to
/dev/ttyS0.

starts a forwarder that accepts connections on port 2022, and directs them
through the proxy daemon listening on port 3128
(proxyport) on host proxy, using the
CONNECT method, where they are authenticated as "user" with "pass" (proxyauth). The proxy
should establish connections to host www.domain.org on port 22 then.

socat - SSL:server:4443,cafile=server.crt,cert=client.pem

is an OpenSSL client that tries to establish a secure connection to an SSL
server. Option cafile specifies a file that
contains trust certificates: we trust the server only when it presents one of
these certificates and proofs that it owns the related private key.
Otherwise the connection is terminated.
With cert a file containing the client certificate
and the associated private key is specified. This is required in case the
server wishes a client authentication; many Internet servers do not.
The first address ('-') can be replaced by almost any other socat address.

is an OpenSSL server that accepts TCP connections, presents the certificate
from the file server.pem and forces the client to present a certificate that is
verified against cafile.crt.
The second address ('PIPE') can be replaced by almost any other socat
address.
For instructions on generating and distributing OpenSSL keys and certificates
see the additional socat docu socat-openssl.txt.

echo |socat -u - file:/tmp/bigfile,create,largefile,seek=100000000000

creates a 100GB sparse file; this requires a file system type that
supports this (ext2, ext3, reiserfs, jfs; not minix, vfat). The operation of
writing 1 byte might take long (reiserfs: some minutes; ext2: "no" time), and
the resulting file can consume some disk space with just its inodes (reiserfs:
2MB; ext2: 16KB).

socat tcp-l:7777,reuseaddr,fork system:'filan -i 0 -s >&2',nofork

listens for incoming TCP connections on port 7777. For each accepted
connection, invokes a shell. This shell has its stdin and stdout directly
connected to the TCP socket (nofork). The shell starts filan and lets it print the socket addresses to
stderr (your terminal window).

functions as primitive binary editor: it writes the 4 bytes 000 014 000 000 to
the executable /usr/bin/squid at offset 0x00074420 (this is a real world patch
to make the squid executable from Cygwin run under Windows, actual per May 2004).

socat - tcp:www.blackhat.org:31337,readbytes=1000

connects to an unknown service and prevents being flooded.

socat -U TCP:target:9999,end-close TCP-L:8888,reuseaddr,fork

merges data arriving from different TCP streams on port 8888 to just one stream
to target:9999. The end-close option prevents the child
processes forked off by the second address from terminating the shared
connection to 9999 (close\(2) just unlinks the inode which stays active as long
as the parent process lives; shutdown\(2) would actively terminate the
connection).

transfers data from stdin to the specified multicast address using UDP. Both
local and remote ports are 6666. Tells the interface eth0 to also accept
multicast packets of the given group. Multiple hosts on the local network can
run this command, so all data sent by any of the hosts will be received
by all the other ones. Note that there are many possible reasons for failure,
including IP-filters, routing issues, wrong interface selection by the
operating system, bridges, or a badly configured switch.

socat TCP:host2:4443 TUN:192.168.255.1/24,up

establishes one side of a virtual (but not private!) network with host2 where a
similar process might run, with UDP-L and tun address 192.168.255.2. They can
reach each other using the addresses 192.168.255.1 and 192.168.255.2. Note that
streaming eg. via TCP or SSL does not guarantee to retain packet boundaries and
may thus cause packet loss.

socat PTY,link=/var/run/ppp,rawer INTERFACE:hdlc0

circumvents the problem that pppd requires a serial device and thus might not
be able to work on a synchronous line that is represented by a network device.
socat creates a PTY to make pppd happy, binds to the network
interfacehdlc0, and can transfer data between
both devices. Use pppd on device /var/run/ppp then.

creates a simple HTTP echo server: each HTTP client that connects gets a valid
HTTP reply that contains information about the client address and port as it is
seen by the server host, the host address (which might vary on multihomed
servers), and the original client request.

waits for an incoming UDP packet on port 9999 and prints the environment
variables provided by socat. On BSD based systems you have to replace
ip-pktinfo with ip-recvdstaddr,ip-recvif. Especially interesting is
SOCAT_IP_DSTADDR: it contains the target address of the packet which may be a
unicast, multicast, or broadcast address.

DIAGNOSTICS

Socat uses a logging mechanism that allows to filter messages by severity. The
severities provided are more or less compatible to the appropriate syslog
priority. With one or up to four occurrences of the -d command line option, the
lowest priority of messages that are issued can be selected. Each message
contains a single uppercase character specifying the messages severity (one of
F, E, W, N, I, or D)

FATAL:

Conditions that require unconditional and immediate program termination.

ERROR:

Conditions that prevent proper program processing. Usually the
program is terminated (see option -s).

WARNING:

Something did not function correctly or is in a state where
correct further processing cannot be guaranteed, but might be possible.

NOTICE:

Interesting actions of the program, e.g. for supervising socat in some kind of server mode.

INFO:

Description of what the program does, and maybe why it
happens. Allows to monitor the lifecycles of file descriptors.

DEBUG:

Description of how the program works, all system or library calls and their results.

Log messages can be written to stderr, to a file, or to syslog.

On exit, socat gives status 0 if it terminated due to EOF or inactivity
timeout, with a positive value on error, and with a negative value on fatal
error.

FILES

/usr/bin/socat
/usr/bin/filan
/usr/bin/procan

ENVIRONMENT VARIABLES

Input variables carry information from the environment to socat, output
variables are set by socat for use in executed scripts and programs.

In the output variables beginning with "SOCAT" this prefix is actually replaced
by the upper case name of the executable or the value of option
-lp.

SOCAT_DEFAULT_LISTEN_IP (input)

(Values 4 or 6) Sets the IP version to
be used for listen, recv, and recvfrom addresses if no
pf (protocol-family) option is given. Is
overridden by socat options -4 or -6.

SOCAT_PREFERRED_RESOLVE_IP (input)

(Values 0, 4, or 6) Sets the IP
version to
be used when resolving target host names when version is not specified by
address type, option pf (protocol-family), or
address format. If name resolution does not return a matching entry, the first
result (with differing IP version) is taken. With value 0, socat always selects
the first record and its IP version.

SOCAT_FORK_WAIT (input)

Specifies the time (seconds) to sleep the
parent and child processes after successful fork\(). Useful for debugging.

SOCAT_VERSION (output)

Socat sets this variable to its version string,
e.g. "1.7.0.0" for released versions or e.g. "1.6.0.1+envvar" for
temporary versions; can be used in scripts invoked by socat.

SOCAT_PID (output)

Socat sets this variable to its process id. In case
of fork address option, SOCAT_PID gets the child processes
id. Forking for exec and system does
not change SOCAT_PID.

SOCAT_PPID (output)

Socat sets this variable to its process id. In
case of fork, SOCAT_PPID keeps the pid of the master process.

SOCAT_PEERADDR (output)

With passive socket addresses (all LISTEN and
RECVFROM addresses), this variable is set to a string describing the peers
socket address. Port information is not included.

SOCAT_PEERPORT (output)

With appropriate passive socket addresses
(TCP, UDP, and SCTP - LISTEN and RECVFROM), this variable is set to a string containing the
number of the peer port.

SOCAT_SOCKADDR (output)

With all LISTEN addresses, this variable is
set to a string describing the local socket address. Port information is not
included example

With all RECVFROM addresses where address
option so-timestamp is applied, socat sets this
variable to the resulting timestamp.

SOCAT_IP_OPTIONS (output)

With all IPv4 based RECVFROM addresses where
address option ip-recvopts is applied, socat fills
this variable with the IP options of the received packet.

SOCAT_IP_DSTADDR (output)

With all IPv4 based RECVFROM addresses where
address option ip-recvdstaddr (BSD) or
ip-pktinfo (other platforms) is applied, socat sets
this variable to the destination address of the received packet. This is
particularly useful to identify broadcast and multicast addressed packets.

SOCAT_IP_IF (output)

With all IPv4 based RECVFROM addresses where
address option ip-recvif (BSD) or
ip-pktinfo (other platforms) is applied, socat sets
this variable to the name of the interface where the packet was received.

SOCAT_IP_LOCADDR (output)

With all IPv4 based RECVFROM
addresses where address option ip-pktinfo is applied,
socat sets this variable to the address of the interface where the packet was
received.

SOCAT_IP_TOS (output)

With all IPv4 based RECVFROM addresses where
address option ip-recvtos is applied, socat sets this
variable to the TOS (type of service) of the received packet.

SOCAT_IP_TTL (output)

With all IPv4 based RECVFROM addresses where
address option ip-recvttl is applied, socat sets this
variable to the TTL (time to live) of the received packet.

SOCAT_IPV6_HOPLIMIT (output)

With all IPv6 based RECVFROM addresses
where address option ipv6-recvhoplimit is
applied, socat sets this variable to the hoplimit value of the received packet.

SOCAT_IPV6_DSTADDR (output)

With all IPv6 based RECVFROM
addresses where address option ipv6-recvpktinfo
is applied, socat sets this variable to the destination address of the received
packet.

SOCAT_IPV6_TCLASS (output)

With all IPv6 based RECVFROM addresses
where address option ipv6-recvtclass is applied,
socat sets this variable to the transfer class of the received packet.